BOSTON (Reuters) - The plan called for John Martorano, a mobster who has confessed to 20 murders, to lure associate Tommy King into a waiting car with a contrived story that he was needed for a hit. King was handed a gun loaded with blanks.

"Pretty much after we pulled out, I shot Tommy," Martorano, 72, told a Boston jury on Tuesday. "Where did I shoot him? In the head."

James "Whitey" Bulger hatched the plot, according to Martorano, who took the stand as a key prosecution witness in the federal trial of Bulger on charges he committed or ordered 19 murders while running Boston's Winter Hill Gang in the 1970s and '80s.

King was targeted in 1975 because Bulger feared he was talking too much. His was one of a dozen murders Martorano calmly recalled to the jury, implicating his former boss, including one in which Bulger allegedly pulled the trigger.

Bulger, now 83, killed many rivals as he rose from small-time crook in a gritty Boston neighborhood to one of the most feared criminals in the city's history, prosecutors charge, but then disappeared and spent 16 years in hiding before his arrest in California 2011. His story has captivated the city for years.

He has pleaded not guilty to all charges.

Martorano, who spent 12 years in prison for the 20 murders to which he confessed, told the jury that he, Bulger and their partner Stephen "The Rifleman" Flemmi regularly teamed up to murder rivals, sometimes accidentally gunning down bystanders.

Bulger played a variety of roles in their attacks, sometimes driving the car carrying the gunman or a support vehicle and sometimes helping to dispose of bodies.

Martorano recalled passing a bridge south of Boston with Bulger, who alluded to King's murder.

"Tip your hat, Tommy's over there," Martorano recalled Bulger as saying. King's body was discovered in the marsh beneath the bridge in 2000.

GUNNED DOWN IN A PHONE BOOTH

Bulger's story inspired the 2006 Academy Award-winning movie "The Departed," and the trial stirred memories of a darker time in Boston's history - some of the killings described occurred just blocks from the waterfront federal courthouse that is the scene of what is expected to be a three- to four-month trial.

The gangster, who earned the nickname "Whitey" for the shock of light hair he sported in his youth, was quick to turn on associates he believed were talking too much about their criminal doings, Martorano said.

Also in 1975, Martorano recalled Bulger and Flemmi gunned down Dorchester bar owner Edward Connors because he believed he was bragging too loudly about helping them with another killing.

"They took him out in the phone booth," said Martorano, who appeared dressed in a dark suit with a red pocket handkerchief and spoke softly. "I heard the shots. They came back and said, 'He's gone.'"

Bulger's willingness to kill associates he suspected of talking too much belied the fact that for years he traded information with a corrupt FBI agent, according to federal prosecutors. Bulger, through his attorneys, denies having been an informant.

But Martorano said the news that his former boss worked with law enforcement "broke my heart" and prompted him to break the gang's code of silence and testify, a deal that allowed him to secure a lesser sentence for his many killings.

J.W. Carney, Bulger's lead attorney, has worked to discredit the government's witnesses, saying they were ready to lie to reduce their time in prison. On Tuesday he called out Martorano's long history of deception in his criminal career.

"I think anyone who has to kill anyone has to lie," Martorano said during cross-examination.